the essence of really bad fan fic
So, fan fic. 
Well yes but OK let's try a different tack here:
What makes a film riffable? And why is it that comedies aren't really riffable? Because comedies are intentionally written to amuse, while the riffable movies are, ususally, created with the deadly serious intention of making a good movie.
Fan Fic writers are, for the most part, deadly serious about what they are writing; they think they're creating some worthwhile, something beautiful or significant to add to the fictional universe(s) they so enjoy.
Which is why the dreck they actually produce is so entertaining. The same impulse that makes some of us like MST3K and Rifftrax and other enterprises like that is what makes us enjoy really terrible fan fiction.

Nobody want seriously written fan fic in this competition, it's a exercise in humorous writing.